In recent years, image display apparatuses such as liquid crystal display apparatuses each including a light source and a light modulation device to modulate the intensity of light from the light source have been widely spread. In these image display apparatuses, however, the degree of adaptation of the human eye (hereinafter referred to as the “adaptation amount of the eye”) to the light intensity in the surroundings of each image display apparatus is not taken into consideration.
In general, the adaptation amount of the eye becomes larger as the ambient illuminance becomes higher, and becomes smaller as the ambient illuminance becomes lower. Also, the differences in luminance between bright objects are more easily recognized where the adaption amount of the eye is larger, and the differences in luminance between dark objects are more easily recognized where the adaptation amount of the eye is smaller. Therefore, when the illuminance in a circumstance where an image display apparatus is viewed suddenly changes, the visual contrast and gradation appear to be lower, though there are no changes in the display characteristics of the image display apparatus.
To restrain such decreases in visual contrast and gradation, there has been a suggested method by which modulation of the luminance of the light source, and a gradation conversion for the respective pixels of an input video image (i.e., a gamma conversion) are performed in accordance with the adaptation amount of the eye.
For example, according to JP-A 2006-285064, when the illuminance suddenly becomes lower, the adaptation amount of the eye is calculated through a predetermined low pass filter operation, and modulation of the luminance of the light source and the gamma conversion are collectively performed in accordance with the adaptation amount of the eye.
However, if the illuminance in a circumstance where an image display apparatus is viewed becomes higher, the illuminance of light incident on the display screen of the image display apparatus becomes higher, resulting in decreases in visual contrast and gradation due to surface reflection. Also, the temporal changes in visual contrast and gradation with respect to the change in the illuminance at this point are much sharper than temporal changes due to adaptation of the eye.
In JP-A 2006-285064, such decreases and temporal changes in visual contrast and gradation due to surface reflection are not taken into consideration. Therefore, after the light intensity in the surroundings changes suddenly, images with improper contrast and gradation are displayed for a while.